The use of anhdyrous ethanol (99.5 to 99.8 vol.% ethanol) has become an important consideration as a means of saving gasoline produced from high-cost crude oil. It is well-known that up to 20 percent anhydrous ethanol can be blended with gasoline to obtain a relatively high octane antiknock fuel which can be used for internal combustion engines. With some engine modification, either anhydrous or hydrous ethanol can be used as the fuel directly.
Growing requirements for anhydrous ethanol for use in motor fuel gasoline blends require systems that operate with minimum energy consumption and that are also reliable in continuous operation. Although blending of ethanol with gasoline has been practiced commercially to some extent during the past forty years, the use of ethanol in such blends has been limited because of the relatively high costs of production.
The conventional distillation system for recovering motor fuel grade anhydrous ethanol from a dilute feedstock, such as fermented beer or synthetic crude alcohol, utilizes three towers, each operated at substantially atmospheric pressure and separately heated with steam. In the first tower the feedstock containing, for example, 6 to 10 vol.% ethanol is subjected to a preliminary stripping and rectifying operation in which the concentration of water is materially reduced. The overhead vapors are condensed at atmospheric pressure with cooling water. A portion of the condensate is returned to the first tower as reflux, and the balance is withdrawn as a concentrated ethanol stream containing on the order of 95 vol.% ethanol, thereby approaching the ethanol-water azeotrope composition of about 97 vol.% ethanol. The concentrated ethanol stream is next subjected to azeotropic distillation in the second or dehydrating tower using a suitable azeotropic or entraining agent, usually benzene or a benzene-heptane mixture. This results in removal of most of the remaining water, and the desired motor fuel grade anhydrous ethanol product is recovered as a bottoms product from the dehydrating tower. The third tower of the system comprises an azeotropic agent stripping tower in which the azeotropic agent is recovered from the water-rich phase following condensation and decantation of the azeotropic overhead stream from the dehydrating tower.
A key factor in the high operating cost of the above-described conventional distillation system is the high thermal energy requirements of the system, particularly steam consumption. Certain proposals have been made in the prior art to reduce the thermal energy requirements of the conventional system. For example, in 1931-32 the Ricard et al U.S. Pat. Nos. 1,822,454 and 1,860,554 disclosed the use of higher pressures in the first tower than in the other towers and the condensation of the high pressure overhead vapors from the first tower to supply heat to the other towers. In the Katzen et al U.S. Pat. No. 4,217,178 even further savings are obtained by combining the multi-pressure level and heat re-use concept with a particular feedstock preheating sequence. However, the energy savings which can be realized by such prior art proposals sometimes fall short of the economies required under present day conditions of high energy costs. Moreover, pressurized operation of the first tower is not always feasible because of its harmful effects on soluble or insoluble components of the feedstock.